• # EEVblog #140 – Battery Capacity Tutorial

Ever wanted to know what battery capacity is? Dave takes you through everything you need to know about amp-hours, mAh, watt hours, internal or series resistance, temperature effects, battery cutoff voltages, and characteristic curves.
Part 2 coming soon.

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• http://chrisgammell.com Chris Gammell

Woo! Amp Hours!

• Jacky Hicks

Excellent and very interesting video. Thank you so much Dave. Now I just need to found the datasheets of all the battery type/brand I use and have a good read, enlighted by your explanations. Again, thank you.

• Andy

I complained last week because Dave doesn’t have any mammary glands.

This week’s vid was superb. I was even able to live with the suck it and see….

• PedroV

Thank you very much Dave! This is really good. Waiting here for part 2! Cheers

• http://www.go-ev.co.uk Greg Fordyce

Great show Dave. My interest is in electric vehicles and am working on my 3rd conversion. I’m glad you mentioned Peukert’s equation, most people in the EV and home power areas are very aware of it. I played around with Peukert’s formula a couple of years back and noticed that it didn’t seem to accurately describe the effect of reduced battery capacity as the discharge rate increased, more of a general ball park figure. I was using a data sheet from a sealed AGM lead acid battery manufacturer that had very detailed charts showing amp hour values for various rates of discharge. In theory you should be able to take any 2 values, calculate the peukert number for the battery and then use the formula to work out the capacity at different discharge rates, but it doesn’t work. If you calculate the Peukert value using the C/20 and C/10 rate and then calculate it again using the C/2 and C/1 rate you get a different Peukert number. So either the formula doesn’t work, I am doing the maths wrong, or the manufactures data sheet is lying. All very possible.

Another thing that interests me about Peukert’s formula is that different battery chemistrys are affected to a greater or lesser extent. For example rechargable lithiums used in electric vehicles have a very low Peukert number, low enough that most people just ignore it and track amp hours in and out of the battery. So now we get back to the point of battery capacity reducing as the discharge gets higher. This is affected by IR of the battery and peukert tries, in my view unsuccessfully, to give a formula for it. But what is Peukert really trying to show. You mentioned that internal resistance of a cell is 2 separate resistances in series, electrical resistance and ionic resistance. A really good lead acid AGM battery has a very low IR, capable of lots of power, yet suffers from the peukert effect. Lithium cells used in EVs have a higher IR, can’t deliver as much power, but can store more WH/kg. I am thinking that Lithium cells have a lower ionic resistance than lead acid and so suffer less from the peukert effect, but their electrical resistance is not (yet) as good. So could it be that the Peukert formula needs to be modified to account for both of these different resistances in the cells in order for it to be more than a ball park guide line?

Sorry for the long post,

Greg.

P.S. battery data sheet I was using is at http://www.secbattery.com/content/images/articles/MICROLYTE%20Red%20Top%20Cycling%20Brochure%20-%2023%20Nov.%202010..pdf

• http://www.eevblog.com EEVblog

I don’t think you are doing your calculations wrong, Peukert’s formula isn’t the be-all end-all.
I’m sure more complex empirical formula could be derived for any given battery technology.
I mentioned it to show that it is possible to derive basic formula to try and model the losses.

Dave.

• Fritz

Incredible amount of energy? For some perspective Gasoline weighing the same as an AA alkaline cell contains about 1.3MJ of chemical energy. About 140 times more than alkaline cells by weight. Kinda explains why electric vehicles are always going to suck.

With all this battery insight what did you work out the life of an AA alkaline in your #139 vblog would be?

• http://www.eevblog.com EEVblog

Incredible amount of energy?

Compared the what most people think, yes.
Why stop at gasoline?, E=MC²
Your kids won’t be saying the same thing when they start to see gasoline run out.

It’s not just a AA Alkaline in my particular project for #139, I made the mistake of limiting the mention to just Alkaline AA because people are familiar with them, and ignoring the IR for the purposes of the discussion. Then the nit-pickers started.

• iluvtocnc

I propose that Dave and Chris’s radio show should be changed to “The Watt Hour”, because “The Amp Hour” discriminates against voltage.

• Jay K

Dave, good topic. I have to wonder why so many hand-held multimeter manufacturers are in love with 9v batteries. An alkaline 9v battery has about 20% more energy than a single 1.5V AA alkaline, but costs almost 3 times as much! And yes, Fluke, I’m looking directly at you! The cost of a boost converter for Fluke is probably less than the cost of a single 9v battery for me. Death to 9v batteries.

• http://www.eevblog.com EEVblog

It’s an interesting question.
I think it generally comes down to that it’s easier (and cheaper) to generate any required voltages from a 9V battery than from 2 x AA’s.
e.g. most multimeter with AA’s will have very low diode test voltages because they don’t want to build in the DC-DC converter to allow for higher voltages.

• Nick

One more reason to avoid using only 2x AA batteries in a multimeter, would be that with the boost converter you would have more electrical noise to fight with at your measurements…

• Jay K

Nick, Fluke and other multi-meter makers use both 2x AA and single 9v, so obviously they can be powered either way. My point is that 2x AA have about 67% more capacity than a 9v, yet cost 33% less. I may put \$100 worth of 9v batteries in a DMM over its lifetime, so I definitely look at the type of batteries they use. IMO, there’s no excuse to use 9v batteries in an expensive meter.

• http://alihassanelashmawy.blogspot.com/ ali hassan

WoW, it is a great blog, i will follow it regularly.

i have a blog in the same field of electrical engineering with namw as :

Free Engineering School

http://alihassanelashmawy.blogspot.com/

please, take a look and advise me how to develop it to become as yours

thanks,

• http://www.rodneykeeling.com Rodney Keeling

GREAT tutorial. You should definitely consider making more tutorials!

• flex

nice video!

a small nag,
@3.00, you say power=V*I, Ohm’s law
but that isn’t Ohm’s law.
just sayin’

• MG

OH MY GOD!!! IT’S MORE THAN 9000!!! (sorry, someone had to say it :))

• Mike

Dave,
Good video. I’m not an engineer or a scientist and I think you did a terrific job of explaining a very interesting aspect of battery capacity for a newbie like me to the field of electronics.
Mike

• Ian

A way to visualize the energy content of a battery – express the energy content as the potential energy of a mass in the Earth’s gravitational field.

E = mgh where E is in Joules, m in kilograms and height h in metres. g in these units is 9.81.
9000 Joules represents a kilogram 917 metres high, or a 40 lb cannonball at 50 metres. Imagine the hole in the ground that would make when dropped, or pity the poor multimeter on the ground underneath it.

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